A power storage device such as an electric double-layer capacitor is a capacitor which utilizes the electric double-layer phenomenon in which, when contacting a polarizable electrodes with an electrolytic solution, counter electric charges are stored on the interface between the surface of the polarizable electrodes and the electrolytic solution, and is generally constituted by a pair of counter polarizable electrodes, a separator separating the pair of the polarizable electrodes electrically and physically, and an organic electrolytic solution. Active carbon powder or the like having a large charge storage interface, namely, a large specific area is used as the polarizable electrodes.
Since the electric double-layer capacitor has a large electrode area and can give a considerably large capacity in comparison with an aluminum electrolytic capacitor which is recognized as having a large capacity among capacitors, there has been used mainly for a memory back-up purpose of electric home appliances, and the like.
Recently, attention is paid to the electric double-layer capacitor having a large capacity and its use is spreading to various applications such as cars, solar power generation and wind power generation in addition to uses for OA devices and industrial machines.
From structural point of view, the electric double-layer capacitor has a coin type, a wound type and a lamination type, and the capacity of the electric double-layer capacitor is determined by a surface area of the electrode acting as a charge storage interface.
The coin-type electric double-layer capacitor has a pair of polarizable electrodes obtained by binding fine active carbon fibers or active carbon powders through the use of a binder to thereby form a mat-shaped material, and then punching the material into a circular shape. In addition, the coin-type electric double-layer capacitor was constituted, by impregnating a separator interposed in parallel between the pair of polarizable electrodes with an electrolytic solution, then storing the polarizable electrodes and the separator in a metal case doubling as an outer casing, and furthermore by sealing the case by swaging a metal lid via a gasket.
In order to enlarge a surface area of an electrode material, the wound-type electric double-layer capacitor has an electrode obtained by coating and binding a fine powdery active carbon onto a surface of a metal foil being a collector through the use of a binder. In addition, the one pair of the electrodes is wound by interposing a separator to form a capacitor element, and the wound-type electric double-layer capacitor is constituted, by storing the element in a metal case, then injecting an electrolytic solution thereinto, and by sealing the case.
In order to enlarge a surface area of an electrode material, the lamination-type electric double-layer capacitor has an electrode obtained by coating and binding a fine powdery active carbon onto a surface of a metal foil being a collector by the use of a binder. In addition, the active carbon electrode and a separator are alternately laminated to form a capacitor element, and the lamination-type electric double-layer capacitor is constituted, by storing the element in a metal case or a multilayered laminate film using a thick aluminum foil, then injecting an electrolytic solution thereinto, and by sealing the case.
There is adopted the wound-type structure or the lamination-type structure of the electric double-layer capacitor having a large capacity whose application is recently spreading. The large capacity type is used for regenerated energy of cars and the like or is used for systems of wind power generation and solar power generation, having large load fluctuation. These uses require properties such as excellent instantaneous charge-discharge, and long cycle life.
In order to enhance the charge-discharge property and achieve the long cycle life of the electric double-layer capacitor, it is essential to realize low resistance of the capacitor. When a large current is charged and discharged for a short time, in a case where an internal resistance value is large, loss due to the resistance is increased. Furthermore, the loss generates heat, and effects of the generated heat accelerates degradation of properties.
For the reduction in the resistance of the electric double-layer capacitor, aggressive improvements of various members such as an electrode material and an electrolytic solution have been carried out, and the reduction in the resistance of the separator is also strongly required.
Effective techniques for reducing the resistance of the separator are to reduce the basis weight of the separator, to reduce the density, and to make the thickness thin.
However, there are caused various problems in mere reduction of the basis weight of the separator, reduction of the density, and making the thickness thin.
The denseness of the separator is also reduced by reduction of the basis weight of the separator, reduction of the density, and making the thickness thin. Accordingly, in a case of the use of the separator as the electric double-layer capacitor, there is a problem that an element short circuit defect rate and an aging short circuit defect rate are increased, and even if short-circuiting does not occur, a leakage current value is increased.
In addition, the value of tearing strength of the separator is also lowered, in a case of reducing the basis weight of the separator, reducing the density, and making the thickness thin. As a result, during the manufacturing step of the electric double-layer capacitor, the separator is broken to lower productivity and yield.
For these reasons, even if the separator has a low basis weight, low density and is thin, the separator is required to have high denseness so as not to increase a short circuit defect rate and strength so as to avoid paper breaking in each step.
In order to enhance denseness of the separator and to reduce the short circuit defect rate of the electric double-layer capacitor, there have been known methods in which the thickness of the separator is made large, and a value of CSF (Canadian Standard Freeness) in accordance with JIS P 8121 which indicates a degree of beating of pulp being a raw material is made smaller, thereby enhancing the density.
However, when the thickness of the separator is made large and the density is increased, a resistance value is worsened.
As mentioned above, as to the separator for the electric double-layer capacitor, even though the separator has low resistance, there is required a thin separator capable enhancing yield while being capable of improving the short circuit defect rate and leakage current property.
In addition, also as to a lithium-ion capacitor and a lithium-ion secondary battery, there also is required a separator satisfying these requirements.
In the separator for power storage device, there have been proposed various configurations so far in order to enhance properties (for example, refer to Patent Literature 1 to Patent Literature 4).